Electron emission regulating means



Nov. 19, 1957 R. BRENHOLDT ELECTRON EMISSION REGULATING MEANS Filed Jan. '7, 1948 U a Q "1 IF M v i m n:

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ATTORNEY United States Patent ELECTRON EMISSION REGULATING MEANS Irving R. Brenholdt, Chicago, Ill., assignor, by mesne assignments, to the United States of America as represented by the United States Atomic Energy Commission Application January 7, 1948, Serial No. 834

3 Claims. (Cl. 250-413) The present invention relates to a method and apparatus for regulating the electron emission of a heated filament or cathode and for maintaining such emission at a constant value when so regulated.

This invention is especially applicable to the regulation of the flow of current in a mass spectrometer by providing a convenient means for adjusting the fiow of space current in the mass spectrometer and maintaining the intensity of the flow of current substantially constant at its adjusted value. Other applications of the invention are to the regulation of current flow in ion gauges and other space discharge devices.

A mass spectrometer is a device adapted to create ions from a gaseous substance fed into it, to accelerate such ions by passing them through a drop in electrical potential and to direct them into a magnetic field in which ions having the same mass follow the same path. Thus, due to the influence of the magnetic field, the ions divide themselves into groups. Each group contains only ions of the same mass and each group follows a different path. A mass spectrometer may be adjusted, usually by changing its ion-accelerating potential, to collect or measure the effect of ions having a given mass and to exclude other ions from the collecting means or detecting electrode or plate. The ions impinging upon the collecting means or plate will cause a flow of current which may be amplified and used as a measure of the number of ions impinging thereon. Accordingly, it is feasible to employ the mass spectrometer as a gas analyzing means provided that the number of ions involved is proportional to the number of molecules from which these ions were produced. The number of ions formed in the mass spectrometer from a given number of molecules depends to a large extent upon the nature of the molecules subjected to the stream of electrons, and upon the number of electrons used in bombarding the molecules to form the ions. Accordingly, it is possible to make the number of ions of a given gas or gaseous mixture proportional to the molecules from which they were formed if the electron emission from the filament is maintained at a constant value. Several factors affect the number of electrons emitted by a heated filament or cathode. Naturally the temperature of the filament is most important; but normally this condition is employed as the controlling means for regulating the number of electrons emitted and usually may be controlled fairly easy. Some of the other factors are most troublesome and tend to cause the electron emission to vary in an uncontrolled manner. These other factors include the character of the gas surrounding the filament, the gas pressure, and the shape, size and material from which the cathode is formed. In a mass spectrometer most of these conditions would vary more or less irregularly and would cause considerable variance in the electrons emitted unless means were provided for holding such emission constant irrespective of changing conditions.

While the effects of changing electron emission, and the reasons for requiring regulation of electron emission 2,813,978 Patented Nov. 19, 1957 have been described in connection with the operation of a mass spectrometer, it is apparent that similar observations might have been made about the performance of ion gauges and other space discharge devices.

Accordingly, it is a principal object of the present invention to provide a method and an apparatus for insuring substantially constant electron emission from a heated filament or cathode despite irregularly varying conditions which might otherwise make such emission unpredictable and changeable.

It is another object of my invention to provide a method and an apparatus of the above-mentioned general nature which is particularly adapted for use in connection with a mass spectrometer or other suitable electric discharge device, and which, accordingly, is especially sensitive and elfective so that any tendency of the electron emission to change immediately causes responsive action for regulating the emission.

A still further object of applicants invention is the provision of a system responsive to cathode emission of a space discharge device for controlling the fiow of heater current to a cathode by altering the load on the power supply for such cathode.

A still further object of applicants invention is the provision of a system for supplying power to a cathode of a mass spectrometer and for measuring the total emission from the cathode, as well as the flow of current through the trap.

A somewhat more specific object of the invention is to provide an electronic regulating means which employs a sensitive vacuum tube electrically connected with the circuit in which the emitted electrons flow and which causes a load to be applied to a source of energy so that the heating current supplied to the cathode or filament is inversely proportional to the load and thereby varies inversely with the number of electrons emitted.

It is still another object of my invention to provide practical and effective apparatus of the foregoing general character, which apparatus is relatively simple as compared with other devices of this nature, which may be formed of readily procurable elements, which is rugged and effective in operation, and which produces satisfactory results with a minimum of servicing and attention.

Other objects and advantages of this invention will be in part obvious and will in part appear from a consideration of the following specification and appended claims, reference being bad to the accompanying drawing in which the single figure comprises a diagrammatical representation of an electronic circuit embodying one form of the present invention.

Structure and operation in general As pointed out hereinbefore, the method and apparatus of the present invention is particularly useful in regulating the quantity of electrons emitted by the filament of a mass spectrometer and for maintaining such emission constant. While my method and apparatus may be used to advantage generally in connection with the control of electron emission of space discharge devices, it will be described chiefly in connection with a mass spectrometer.

Referring to the drawing the reference numeral 10 indicates the filament of the mass spectrometer or other heated filament or cathode for which it is desired to regulate the emission of electrons. The filament 10 obtains its heating current from the secondary winding 25 of the transformer 8. The primary winding of this transformer 8 has connected in series with it, impedances 47 and 48, the latter being adjustable for reasons more fully discussed hereinafter. It will be observed that the trans former 8 is provided with an additional secondary winding 46 coupled to rectifiers 45, 45' which serve as a variable load when permitted to conduct. When the secondary 46 is thus loaded the primary of transformer 8 draws more current. Increased current flow in the primary circuit develops increased voltage drops across impedances 47 and 48 and lowers the terminal voltage applied to the primary of transformer 8. This in turn results in the lowering of the output voltage across the secondary winding 25 of the transformer which reduces the current flow through cathode 10 of the mass spectrometer. Accordingly, as the loading of the transformer changes, the voltage of the secondary is altered, and the heater current through the cathode 10 likewise changes. In other words, the greater the load provided by the secondary winding 46 the less will be the current delivered by the filament heating secondary coil 25 and vice versa.

A second power transformer 8' is connected to a rectifier-filter system, shown generally in the dotted line inclosure at 5, with the conventional rectifying tube indicated at 4. The purpose of this system is to supply direct current operating voltages for a regulator tube 9 to be further discussed. A secondary winding 39 is also provided on this transformer 8' for supplying filament current to a pair of triodes 45, 45' connected as the variable loading means shunting the secondary coil 46 of the transformer 8 previously mentioned. The center taps of the secondary coils 46 and 39 are connected together.

The mass spectrometer source is only diagrammatically shown and is generally designated 55. It comprises an outer shield or inclosure 56 with a cathode or filament 10, and an anode or trap 11, positioned in spaced relation therein. Electron emission passes from the cathode to the trap with some electrons being collected by the supporting structure and shield.

Very generally speaking, conventional or positive currents flowing in the circuit including the shield and the trap traverse a common path to point 16 of resistance 54. In this path the currents pass through a meter 23 and a series of resistances including the grid load resistor 21 of the regulator tube 9 from the center tap of the filament heating secondary coil 25.

At a suitable point in the system, preferably at 29, there is connected the control grid 28 of the regulator tube 9. The cathode 30 of the regulator tube 9 is provided with a predetermined stabilized voltage resulting from the voltage regulating tube 32. As more fully discussed hereinafter, when the voltage drop across the resistance network shown at 21 reaches a predetermined value as a result of the fiow of cathode emission current through the circuit, the regulator tube 9 will begin to conduct. This takes place because the voltage drop from the point 31 to the point 19 is the cathode potential which is fixed at some desired value, whereas the voltage drop from the point 29 to the point 19 is the grid potential, and the latter will build up a value such that the necessary gridcathode bias is established in the tube 9 causing it to conduct. When the regulator tube 9 becomes conductive, a change in voltage drop appears across its load resistance 13. It will be observed that the triodes 45 and 45', connected to the loading secondary coil 46 for regulating the current in the filament secondary coil 25, are provided with a fixed grid potential as a result of the tie line 43 and the operation of the voltage regulating tubes 44 and 32. Thus the triodes 45 and 45' are provided with an initial or so-called fixed voltage bias between the grid and the cathode. The anode load resistance 13 of the regulator tube 9 changes this so-called fixed bias in accordance with anode current flow whereby the grids of the tubes 45 and 45 become more negative or more positive with respect to their cathodes and either increase or decrease the load on the secondary coil 46 of the transformer 8. In fact, the greater the How of current from the regulator tube 9 through the line 35, the more negative the cathode of the tubes 45 and 45 become. Accordingly, increased current flows across the tubes 45 and 45 between filament or cathode and plate, resulting in greater loading of the secondary coil 46. As pointed out hereinbefore, such loading of the coil 46 will decrease the current delivered by the secondary or heating coil 25 to the filament 10. Accordingly, as the quantity of electrons emitted from the filament 10 increases, the flow of electron current through the circuit increases. When this flow becomes greater than desired, the tube 9 will conduct more current, causing the triodes 45 and 45' to also conduct more current so that more load is placed on secondary coil 46. This load cuts down the heating current supplied to the filament, and therefore cuts down and regulates the emission of the filament in the manner heretofore indicated. The manner of connecting the regulator tube 9 in relation to the resistance network 21 through which the electron current of the mass spectrometer passes is especially advantageous since the regulator tube 9 operates on the linear portion of its characteristic curve and provides maximum amplification under regulating conditions.

The foregoing descriptive portions of the present specification have indicated in a general way only the elements of my electron emission regulating means and their operation. Details of the circuit may be more readily presented and therefore more easily understood as a result of an explanation of the operation of the circuit to hold the electron fiow constant while permitting the flow to be regulated at any desired value within wide limits. Such a detailed explanation of the operation of the circuit follows.

Operation of the circuit for regulating the number of electrons emitted In order to explainthe regulation of the cathode emission of the filament 10, it is desirable first to consider the circuit or path which the emission current follows throughout the circuit. in this connection it should be carefully observed that reference is made to cathode emission current flow. The heated filament indicated at 10 emits electrons, most of which are collected by the trap in the mass spectrometer indicated at 11, but some of which are collected by the shielding mechanism and supports for the spectrometer which are referred to herein as the shield 56. However, in tracing the current flow, the conventional practice of positive curent will be followed, although this is opposite in direction to the actual electron flow. When the tandem switch 57 is in the a position, space current reaching the cathode 10 flows through lines 26, 27 to the transformer winding 25, out the center tap, and back along center line 24 through meter 23 via contacts a, a to line 22, variable resistance 21, and lead 20, to the moving contact on voltage divider 18, then to point 19, line 49, to the center tap of one secondary of power transformer 8', through rectifier 4 to point 14, and thence to resistance 54, where it divides at point 16 with part returning through line 12 and contact a of switch 57 to trap 11. The other passes through line 25a back to the shield 56. When the switch 57 is in the b, b position, current flowing from the cathode 10 passes back through switch 57 to line 22, resistors 21 and 18, line 49, rectifier 4, and resistor 54, to point 16, where it again branches and part passes through line 12, meter 23 through contacts b, b of switch 57, and back to the trap 11. The balance returns through line 25a back to the shield 56. In the a, a position of switch 57 the entire emission current of the cathode of the spectrometer is read on meter 23 while in the b, b position only the current in the trap circuit is measured.

Having thus described the flow of current through the circuit, the means for regulating the quantity of current thus flowing becomes more readily understandable. One way of so regulating the current flow comprises the adjustable or variable resistance network shown at 21. The network shown at 21 operates according to Ohms law and simply reduces or increases the resistance in the line as a means of governing the current flowing through the filament 10. However, this adjustment is comparatively crude and is normally employed to give a rough adjustment only of the quantity of current flowing through the circuit.

More delicate, though not necessarily more limited in range for adjusting the electron flow, is the adjustable resistance 18. It is to be realized that changes in the resistance 18 also change the resistance in the electron circuit and therefore tend to regulate the flow of current in the same way that the resistance network 21 does. However, the resistance 18 is so very small as compared with the resistance network 21 that the effect of changing the resistance 18, insofar as it functions as a resistance to the flow of current, is not appreciable; but its adjustment does materially change the flow of current, and accordingly must operate on some different principle. It will be observed that the cathode 30 of the regulator tube 9 is connected to the high potential side of the resistor 17, which is connected in series with resistor 18. The control grid 28 of the regulator tube 9 is connected to the sliding contact of the resistor 18, although a portion of the resistance network 21 is interposed. The difference in potential between the control grid 28 and the cathode 30 is the so-called operating bias of the regulating tube 9. This bias is caused by a somewhat greater voltage drop between the points 31 and 19 than that between the points 29 and 19. Furthermore, it will be observed that the voltage at the point 31 is fixed by means of the voltage regulating tube 32. Therefore, the voltage at the line 20 will depend upon the adjustment of the variable resistance 18 since the resistances 17 and 18 are, in effect, a voltage divider. In other words, adjustment of variable resistance 18 serves to provide some definite voltage at the grid 28. Accordingly, the flow of current through the resistance network 21 causes a certain voltage drop between the points 29 and line 20 depending upon the quantity of emission current flowing. Current fiow through resistance 18 also introduces a drop. The voltage drops must increase finally reaching a point at which the bias in the tube 9 is such as to cause the tube to conduct. As more fully pointed out hereinafter, such condition serves to regulate the flow of electrons in the circuit.

Assume that the potential from cathode 30 to point 19 is 75 v. To make the tube 9 function, a potential of 73 /2 v. (approx) must be developed between the grid 28 and the point 19. This voltage is obtained in two ways: (1) The voltage drop between grid 28 and point 19 as a result of emission current of the spectrometer 55 and (2) a portion of the fixed voltage across resistor 18.

This operation of the adjustable resistance 18 to produce an accurate and wide regulation of the emission current flow is many times more delicate and covers a much greater range than its efiect simply as a resistance in the line. A specific example of operating values is given hereinafter:

In the apparatus shown, the voltage bias in the tube 9 may be set at approximately 1 /2 volts. The value of the bias which will be required depends on a number of factors fixed by the design of the apparatus, such as the number and charge on the other screens in the tube, and the load on the plate, that is, resistance 13. At any rate when this grid cathode bias of 1 volts is reached, the tube will conduct. When the resistance 18 is adjusted as shown, the voltage at the cathode 30 will be approximately 50 volts. This is accomplished by the voltage regulating tube 32 which maintains a constant voltage drop of 75 volts from the point 31 to the point 19. Accordingly the resistances 17 and 18 operate as a voltage divider. Therefore, the arm of potentiometer 18 may be adjusted to provide a voltage drop of 50 volts from the point 31 to the line 20. When the voltage between the point 29 and the line 20 becomes about 48 /2 volts, which will occur as a result of the flow of emission current across the resistances 21, the tube 9 will amplify because: (I) The drop from 31 to 20 is the cathode potential (50 volts), (2) the drop from 29 to 20 is the grid potential (48 /2 volts), (3) this establishes a grid-cathode bias in the tube of 1 /2 volts causing the tube to conduct. If the emission current tries to increase further, it is regulated at this level as a result of this amplification. Referring to the drawing, if the moving contact at resistance 18 is placed at the point 19, the voltage of the cathode 30 will be 75 volts so that the electron emission must increase in order to build up the potential between the point 29 and the point 20 to about 73 /2 colts instead of 48 /2 volts. This, of course, results in a greater number of electrons being emitted from the filament l0. correspondingly, a smaller quantity will be emitted it the voltage on the cathode 30 is decreased. Thus moving the contact toward the point 19 increases electron emission, whereas moving it toward the fixed resistance 17 decreases such emission.

Operation of a circuit for holding the electron emission constant at its regulated value The operation of the regulator tube 9, triodes 45 and 45, and secondary coil 46 of the transformer 8 which are the principal elements in the circuit for holding the cathode emission constant, may best be explained by considering what must take place as the emission tends to increase or to decrease from its regulated value. Assuming first that the emission from the filament 10 tends to increase, it is evident that electrons will be collected by the trap or the shield or both, and current will therefore flow through the circuit in the manner described in detail above. This means, of course, that this increased current fiow will pass through the resistances 21 and 18, thereby, increasing the potential at the grid 28 causing the tube 9 to become more conductive for the reasons explained in detail in the last preceding paragraphs. As a result of this increased conductance of the regulating tube 9, more current will fiow through voltage regulator tube 32 and rectifier 4 to the point 14, and across the plate load resistor 13 to the point 35a, which is the control point for the rectifiers 45, 45'. Due to the increased flow of the current in the circuit described above through the regulator tube 9, point 35a becomes more negative, because the voltage at the point 14 is fixed by the rectifying system shown at 5, and the current flow must cause a definite voltage drop across the resistor 13. It will be noted that the line 36 connects the point 35a with the cathodes 37 and 38 of the triodes 45 and 45' through the center tap 40 of the secondary 39. Therefore, as the current flows across the resistor 13 and through the tube 9, the point 35a and the cathodes 37 and 38 become more and more negative as the flow increases. The grids 41 and 42 of the triodes 45 and 45' are maintained at a constant voltage by means of the line 43 leading to the voltage regulating tube 44. With a constant charge on the grids 41 and 42, as the cathodes 37 and 38 become negative, the tubes 45 and 45 will conduct more and more current in their cathode-plate circuit, and establish a larger and larger flow of current through the secondary winding 46 of the transformer 8. For the reasons already discussed, this fiow of current through the secondary windings and the triodes 45 and 45' results from the conductance of the tube 9 which, in turn, operates as a result of excessive cathode emission from the mass spectrometer. Accordingly, when the electron emission is excessive, a comparatively large flow of current takes place in the secondary loading coil 46. As also discussed hereinbefore, this loading of the coil 46 reduces the current fed by the secondary coil 25 of the transformer 8. Therefore, loading of the coil 46 decreases the current produced by the filament heating coil 25, and the greater the loading the less current produced by the coil 25, so that as the cathode emission becomes excessive the heating current to the filament is immedately decreased and such emission is corrected practically instantaneously.

In this connection it will be noted that the emission regulation system of the present invention is particularly sensitive in operation because the regulating tube 9 may be made responsive to small changes in current in the line at the point 29 and therefore becomes immediately sensitive to changes in the cathode emission. In other words, should the emission try to increase above the regulated value, the regulating tube 9 would immediately send a comparatively large current through the line 35. Also, the rectifying triodes 45 and 45 amplify the resulting change in cathode potential so that a relatively large current passes through the coil 46 in spite of a small change occurring at the point 29. The greater the overall current amplification of these tubes, the more nearly constant will be the regulated emission current.

It will be noted that the switch 52 must be closed in order to actuate the regulating mechanism before the switch 53, which actuates the filament in the spectrometer, is closed. This is to allow the indirectly heated cathode tube 9 to heat up before the emission in the spectrometer proceeds. Filament of tube 9 is energized from source 50 through filament transformer 51. The extremities of the tapped secondary are connected to the cathode heater element of tube 9, and the center tap is connected to the B power source at point 31. However, for purposes of clarity, leads from transformer 51 to the filament of tube 9 have been omitted and indicating symbols x, x have been substituted therefor.

It will be understood that a non-inventive modification of the emission regulating circuit might comprehend the substitution of a transformer for the ballast resistors 47, 48, in which event the excitation current for the spectrometer, ion gauge, or other discharge device would be taken from the secondary of such transformer instead of the secondary 25 of transformer 8. This would further necessitate alteration of the control circuit for tubes 45, 45' to cause them to respond in an inverse manner to that contemplated in the modification disclosed in the drawings.

Having thus described my invention, I claim:

1. A control system for regulating the cathode emission of a space discharge device comprising a mass spectrometer having a heated cathode, a circuit including said cathode, a power transformer for supplying heater current to said cathode, said transformer having a plurality of secondaries, means for loading one of said secondaries, said means including an electric discharge device having its output coupled thereto, and a control space discharge device having its control grid coupled to said circuit and responsive to changes in the emission of said cathode for controlling the operation of the electric discharge device to alter the load on said transformer and regulate the heater current, and means for maintaining the cathode 8 of said control space discharge device at a constant potential.

2. A control system for regulating the cathode emission of a space discharge device comprising a mass spectrometer having a heated cathode, a circuit including said cathode, a power transformer for supplying heater current to said cathode, said transformer having a ballast resistor in its primary and a plurality of secondaries, a pair of electric discharge devices having their output circuits coupled to one of the secondaries for loading it, means for maintaining the control grids of said electric discharge devices at a substantially constant potential, and a control discharge device coupled to said circuit and responsive to changes in emission current therein for applying a positive potential to the cathodes of said electric discharge devices to alter the impedance of their output circuits in accordance with the changes in emission current of said .space discharge device and regulate the flow of heater current through said cathode.

3. A control system comprising a mass spectrometer having a heated cathode, a power transformer for supplying heater current to said cathode, said transformer having a plurality of secondaries, means for loading one of said secondaries, an adjustable resistor in the cathodeanode circuit of the spectrometer, a space discharge device having its input circuit coupled to the adjustable resistor and responsive to emission from said cathode above a predetermined value, means for coupling the output circuit of the discharge device to the loading means to control the operation thereof and adjust the loading of said transformer to reduce the heater current supplied to said cathode, and means for maintaining the cathode of the discharge device at a substantially fixed potential.

References Cited in the file of this patent UNITED STATES PATENTS 1,406,328 Atherton Feb. 14, 1922 1,683,194 Kearsley Sept. 4, 1928 1,953,889 Mutscheller Apr. 3, 1934 2,048,203 Spencer July 21, 1936 2,319,378 Weisglass May 18, 1943 2,544,716 Nier Mar. 13, 1951 OTHER REFERENCES Ridenour et al.: Review of Scientific Instruments, May 1937, vol. 8, pp. 162-164.

Basic Radio by Hoag, published 1942 by D. Van Nostrand Co., Norwood, Massachusetts, by Plimpton Press, pages 83 and 242.

Nier: Review of Scientific Instruments, June 1947, vol. 18, pp. 398-404. 

